This invention generally relates to liquid phase oxidation processes using a carbon-supported, noble-metal-containing catalyst (particularly a deeply reduced catalyst) in conjunction with a supplemental promoter (e.g., bismuth or tellurium). In an especially preferred embodiment, this invention relates to such a process wherein N-(phosphonomethyl)iminodiacetic acid (xe2x80x9cPMIDAxe2x80x9d) or a salt thereof is oxidized to form N-(phosphonomethyl)glycine (also known in the agricultural chemical industry as xe2x80x9cglyphosatexe2x80x9d) or a salt thereof. This invention also generally relates to enhancing the activity, selectivity, and/or stability of a carbon-supported, noble-metal-containing catalyst (particularly a deeply reduced catalyst) using a supplemental promoter.
N-(phosphonomethyl)glycine is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in an aqueous formulation. Glyphosate is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz (U.S. Pat. No. 3,950,402) discloses that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of PMIDA with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support: 
Other by-products also typically form, such as formic acid (HCO2H), which is formed by the oxidation of the formaldehyde by-product; and aminomethylphosphonic acid (xe2x80x9cAMPAxe2x80x9d), which is formed by the oxidation of N-(phosphonomethyl)glycine. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., xe2x80x9cleachingxe2x80x9d) result because, under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form, and both PMIDA and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal.
In U.S. Pat. No. 3,969,398, Hershman discloses that activated carbon alone, without the presence of a noble metal, may be used to effect the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine. In U.S. Pat. No. 4,624,937, Chou further discloses that the activity of the carbon catalyst disclosed by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formic acid and formaldehyde by-product when used to effect the oxidative cleavage of N-phosphonomethyliminodiacetic acid. These byproducts are particularly undesirable because they react with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as xe2x80x9cNMGxe2x80x9d) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Optimally, therefore, it has been suggested that the formic acid and formaldehyde be simultaneously oxidized to carbon dioxide and water as the PMIDA is oxidized to N-(phosphonomethyl)glycine in a single reactor, thus giving the following net reaction: 
As the above references suggest, such a process requires the presence of both carbon (which primarily effects the oxidation of PMIDA to form N-(phosphonomethyl)glycine and formaldehyde) and a noble metal (which primarily effects the oxidation of formaldehyde and formic acid to form carbon dioxide and water). Previous attempts to develop a stable catalyst for such an oxidation process, however, have not been entirely satisfactory.
Like Franz, Ramon et al. (U.S. Pat. No. 5,179,228) disclose using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), however, Ramon et al. disclose flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%. Still, the amount of noble metal loss incurred with this method is unacceptable. In addition, re-depositing the noble metal can lead to loss of noble metal surface area which, in turn, decreases the activity of the catalyst.
Using a different approach, Felthouse (U.S. Pat. No. 4,582,650) discloses using two catalysts: (i) an activated carbon to effect the oxidation of PMIDA into N-(phosphonomethyl)glycine, and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde into carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of PMIDA to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle.
In PCT/US99/03402, Ebner et al. disclose a reaction process for making N-(phosphonomethyl)glycine compounds from PMIDA compounds using a deeply reduced, carbon-supported, noble metal catalyst which exhibits improved resistance to noble metal leaching and increased destruction of undesirable byproducts (e.g., formaldehyde). Still, this reaction process typically does not eliminate all the formaldehyde and formic acid byproduct, and, consequently, also does not eliminate all the N-methyl-N-(phosphonomethyl)glycine byproduct.
Thus, a need continues to exist for an improved reaction process for oxidizing PMIDA to N-(phosphonomethyl)glycine using a catalyst which exhibits resistance to noble metal leaching and increased oxidation of formic acid and formaldehyde into carbon dioxide and water (i.e., increased formic acid and formaldehyde activity).
This invention provides, in part, for an improved process for oxidizing PMIDA, salts of PMIDA, and esters of PMIDA to form N-(phosphonomethyl)glycine, salts of N-(phosphonomethyl)glycine, and esters of N-(phosphonomethyl)glycine, particularly such a process which uses a catalyst (or catalyst system) that (a) exhibits resistance to noble metal leaching, and (b) exhibits increased oxidation of formic acid and/or formaldehyde, and consequent decreased formation of NMG; an improved process for oxidizing a substrate in general wherein the activity, selectivity, and/or stability of a carbon-supported, noble-metal-containing catalyst used to catalyze the oxidation is enhanced by merely mixing the catalyst with a supplemental promoter (rather than using a catalyst which already contains the promoter, and, consequently, is more costly to manufacture); an improved process for making an oxidation catalyst system (particularly an oxidation catalyst system for oxidizing PMIDA compounds) having enhanced activity, selectivity, and/or stability; and an oxidation catalyst system (particularly an oxidation catalyst system for oxidizing PMIDA compounds) having enhanced activity, selectivity, and/or stability.
Briefly, therefore, the present invention is directed to a process for oxidizing formic acid or formaldehyde in the presence of a catalyst and a supplemental promoter. Here, the catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the catalyst is at least about 1:15,000.
The present invention is also directed to a process for oxidizing a substrate in general using a catalyst comprising a carbon support and a noble metal. In this embodiment, the process comprises contacting the substrate with oxygen in the presence of the catalyst and a supplemental promoter. Here, the mass ratio of the supplemental promoter to the catalyst is at least about 1:15,000. And, before the catalyst is used in the oxidation of the substrate, the catalyst:
A. comprises a non-graphitic carbon support having a noble metal at a surface of the non-graphitic carbon support; and
is identifiable as yielding no greater than about 0.7 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst in a helium atmosphere is heated from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or
B. comprises a non-graphitic carbon support having a noble metal and a catalyst-surface promoter at a surface of the non-graphitic carbon support; and
is identifiable as yielding no greater than about 0.7 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst, after being heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or
C. comprises a non-graphitic carbon support having a noble metal, carbon, and oxygen at a surface of the non-graphitic carbon support, the ratio of carbon atoms to oxygen atoms at the surface being at least about 30:1, as measured by x-ray photoelectron spectroscopy; or
D. comprises a non-graphitic carbon support having a noble metal, a catalyst-surface promoter, carbon, and oxygen at a surface of the non-graphitic carbon support; and
is identifiable as having a ratio of carbon atoms to oxygen atoms at the surface which is at least about 30:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere; or
E. comprises a non-graphitic carbon support having (i) a noble metal at a surface of the non-graphitic carbon support; and (ii) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising oxygen and carbon, the ratio of carbon atoms to oxygen atoms in the surface layer being at least about 30:1, as measured by x-ray photoelectron spectroscopy; or
F. comprises a non-graphitic carbon support having: (a) a noble metal and a catalyst-surface promoter at a surface of the non-graphitic carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen; and
is identifiable as having a ratio of carbon atoms to oxygen atoms in the surface layer of at least about 30:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere;
G. is formed by a process comprising depositing a noble metal at a surface of a non-graphitic carbon support, and then heating the surface at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
H. is formed by a process comprising depositing a noble metal at a surface of a carbon support, and then exposing the surface to a reducing environment, wherein, before the noble metal deposition, the carbon support has carbon atoms and oxygen atoms at the surface of the carbon support in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
I. is formed by a process comprising depositing a noble metal at a surface of a non-graphitic carbon support, and then heating the surface at a temperature greater than about 500xc2x0 C.
The present invention is also directed to a process for making an oxidation catalyst system.
In one embodiment directed to a process for making an oxidation catalyst system, the process comprises mixing a noble-metal-containing catalyst with a supplemental promoter in the presence of formic acid or formaldehyde. Here, the noble-metal-containing catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000.
In another embodiment directed to a process for making an oxidation catalyst system, the catalyst system is prepared using a carbon support having carbon atoms and oxygen atoms at a surface of the non-graphitic carbon support. In this process, a noble metal is deposited at the surface of the carbon support to form a noble-metal-containing catalyst. Oxygen-containing functional groups are subsequently removed from the surface of the noble-metal-containing catalyst to form a noble-metal-containing catalyst comprising a deoxygenated surface. This removal of oxygen-containing functional groups comprises:
(i) heating the surface of the noble-metal-containing catalyst at a temperature of greater than about 500xc2x0 C.; or
(ii) heating the surface of the noble-metal-containing catalyst at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
(iii) exposing the surface of the noble-metal-containing catalyst to a reducing environment, wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
(iv) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that the ratio of carbon atoms to oxygen atoms at the deoxygenated surface of the noble-metal-containing catalyst comprising the deoxygenated surface is at least about 30:1, as measured by x-ray photoelectron spectroscopy; or
(v) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that no greater than about 0.7 mmole of carbon monoxide per gram of the noble-metal-containing catalyst comprising the deoxygenated surface desorb from the deoxygenated surface when a dry sample of the noble-metal-containing catalyst comprising the deoxygenated surface is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes.
After removing oxygen-containing functional groups from the surface of the noble-metal-containing catalyst, the noble-metal-containing catalyst is mixed with a supplemental promoter. Here, the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000.
This invention is also directed to an oxidation catalyst system.
In one embodiment directed to an oxidation catalyst system, the oxidation catalyst system is prepared by a process comprising mixing a noble-metal-containing catalyst with a supplemental promoter in the presence of formic acid or formaldehyde. Here, the noble-metal-containing catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000.
In another embodiment directed to an oxidation catalyst system, the oxidation catalyst system is prepared using a carbon support. When preparing this catalyst system, a noble metal is deposited onto a surface of the carbon support to form a noble-metal-containing catalyst. Oxygen-containing functional groups are subsequently removed from the surface of the noble-metal-containing catalyst to form a noble-metal-containing catalyst comprising a deoxygenated surface. This removal of oxygen-containing functional groups comprises:
(i) heating the surface of the noble-metal-containing catalyst at a temperature of greater than about 500xc2x0 C.; or
(ii) heating the surface of the noble-metal-containing catalyst at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the non-graphitic carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
(iii) exposing the surface of the noble-metal-containing catalyst to a reducing environment, wherein, before the noble metal deposition, the non-graphitic carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
(iv) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that the ratio of carbon atoms to oxygen atoms at the deoxygenated surface of the noble-metal-containing catalyst comprising the deoxygenated surface is at least about 30:1, as measured by x-ray photoelectron spectroscopy; or
(v) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that no greater than about 0.7 mmole of carbon monoxide per gram of the noble-metal-containing catalyst comprising the deoxygenated carbon support surface desorb from the deoxygenated surface when a dry sample of the noble-metal-containing catalyst comprising the deoxygenated surface is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes.
After oxygen-containing functional groups have been removed from the surface of the noble-metal-containing catalyst, the noble-metal-containing catalyst is mixed with a supplemental promoter. Here, the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000.
This invention also is directed to a general process for making N-(phosphonomethyl)glycine, a salt of N-(phosphonomethyl)glycine, or an ester of N-(phosphonomethyl)glycine. This process comprises oxidizing N-(phosphonomethyl)iminodiacetic acid, a salt of N-(phosphonomethyl)iminodiacetic acid, or an ester of N-(phosphonomethyl)iminodiacetic acid in the presence of an oxidation catalyst. Before the oxidation, this oxidation catalyst:
A. comprises a carbon support having a noble metal at a surface of the carbon support; and
is identifiable as yielding no greater than about 1.2 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst in a helium atmosphere is heated from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or
B. comprises a carbon support having a noble metal, carbon, and oxygen at a surface of the carbon support, the ratio of carbon atoms to oxygen atoms at the surface being at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
C. comprises a carbon support comprising: (a) a noble metal at a surface of the carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen, the ratio of carbon atoms to oxygen atoms in the surface layer being at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
D. is formed by a process comprising depositing a noble metal at a surface of a carbon support, and then heating the surface at a temperature of at least about 400xc2x0 C.; or
E. is formed by a process comprising:
depositing a noble metal at a surface of a carbon support, and then exposing the surface to a reducing environment, wherein, before the noble metal deposition, the carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or
F. comprises a carbon support having a noble metal, a promoter, carbon, and oxygen at a surface of the carbon support; or
G. comprises a carbon support having: (a) a noble metal and a promoter at a surface of the carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen, the catalyst being identifiable as having a ratio of carbon atoms to oxygen atoms in the surface layer which is at least about 20:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
Other features of this invention will be in part apparent and in part pointed out hereinafter.